linux/Documentation/admin-guide, branch v6.15

x86/its: Add support for RSB stuffing mitigation

2025-05-09T20:22:05Z

When retpoline mitigation is enabled for spectre-v2, enabling call-depth-tracking and RSB stuffing also mitigates ITS. Add cmdline option indirect_target_selection=stuff to allow enabling RSB stuffing mitigation. When retpoline mitigation is not enabled, =stuff option is ignored, and default mitigation for ITS is deployed. Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Reviewed-by: Josh Poimboeuf Reviewed-by: Alexandre Chartre

x86/its: Add "vmexit" option to skip mitigation on some CPUs

2025-05-09T20:22:05Z

Ice Lake generation CPUs are not affected by guest/host isolation part of ITS. If a user is only concerned about KVM guests, they can now choose a new cmdline option "vmexit" that will not deploy the ITS mitigation when CPU is not affected by guest/host isolation. This saves the performance overhead of ITS mitigation on Ice Lake gen CPUs. When "vmexit" option selected, if the CPU is affected by ITS guest/host isolation, the default ITS mitigation is deployed. Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Reviewed-by: Josh Poimboeuf Reviewed-by: Alexandre Chartre

x86/its: Enable Indirect Target Selection mitigation

2025-05-09T20:22:05Z

Indirect Target Selection (ITS) is a bug in some pre-ADL Intel CPUs with eIBRS. It affects prediction of indirect branch and RETs in the lower half of cacheline. Due to ITS such branches may get wrongly predicted to a target of (direct or indirect) branch that is located in the upper half of the cacheline. Scope of impact =============== Guest/host isolation -------------------- When eIBRS is used for guest/host isolation, the indirect branches in the VMM may still be predicted with targets corresponding to branches in the guest. Intra-mode ---------- cBPF or other native gadgets can be used for intra-mode training and disclosure using ITS. User/kernel isolation --------------------- When eIBRS is enabled user/kernel isolation is not impacted. Indirect Branch Prediction Barrier (IBPB) ----------------------------------------- After an IBPB, indirect branches may be predicted with targets corresponding to direct branches which were executed prior to IBPB. This is mitigated by a microcode update. Add cmdline parameter indirect_target_selection=off|on|force to control the mitigation to relocate the affected branches to an ITS-safe thunk i.e. located in the upper half of cacheline. Also add the sysfs reporting. When retpoline mitigation is deployed, ITS safe-thunks are not needed, because retpoline sequence is already ITS-safe. Similarly, when call depth tracking (CDT) mitigation is deployed (retbleed=stuff), ITS safe return thunk is not used, as CDT prevents RSB-underflow. To not overcomplicate things, ITS mitigation is not supported with spectre-v2 lfence;jmp mitigation. Moreover, it is less practical to deploy lfence;jmp mitigation on ITS affected parts anyways. Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Reviewed-by: Josh Poimboeuf Reviewed-by: Alexandre Chartre

Documentation: x86/bugs/its: Add ITS documentation

2025-05-09T20:22:04Z

Add the admin-guide for Indirect Target Selection (ITS). Signed-off-by: Pawan Gupta Signed-off-by: Dave Hansen Reviewed-by: Josh Poimboeuf Reviewed-by: Alexandre Chartre

xfs: remove duplicate Zoned Filesystems sections in admin-guide

2025-04-22T14:05:24Z

Remove the duplicated section and while at it, turn spaces into tabs. Signed-off-by: Hans Holmberg Reviewed-by: Christoph Hellwig Reviewed-by: Carlos Maiolino Fixes: c7b67ddc3c99 ("xfs: document zoned rt specifics in admin-guide") Signed-off-by: Carlos Maiolino

xfs: document zoned rt specifics in admin-guide

2025-04-17T06:16:59Z

Document the lifetime, nolifetime and max_open_zones mount options added for zoned rt file systems. Also add documentation describing the max_open_zones sysfs attribute exposed in /sys/fs/xfs//zoned/ Fixes: 4e4d52075577 ("xfs: add the zoned space allocator") Signed-off-by: Hans Holmberg Reviewed-by: Christoph Hellwig Reviewed-by: Darrick J. Wong Signed-off-by: Carlos Maiolino

xfs: add tunable threshold parameter for triggering zone GC

2025-04-14T08:41:33Z

Presently we start garbage collection late - when we start running out of free zones to backfill max_open_zones. This is a reasonable default as it minimizes write amplification. The longer we wait, the more blocks are invalidated and reclaim cost less in terms of blocks to relocate. Starting this late however introduces a risk of GC being outcompeted by user writes. If GC can't keep up, user writes will be forced to wait for free zones with high tail latencies as a result. This is not a problem under normal circumstances, but if fragmentation is bad and user write pressure is high (multiple full-throttle writers) we will "bottom out" of free zones. To mitigate this, introduce a zonegc_low_space tunable that lets the user specify a percentage of how much of the unused space that GC should keep available for writing. A high value will reclaim more of the space occupied by unused blocks, creating a larger buffer against write bursts. This comes at a cost as write amplification is increased. To illustrate this using a sample workload, setting zonegc_low_space to 60% avoids high (500ms) max latencies while increasing write amplification by 15%. Signed-off-by: Hans Holmberg Reviewed-by: Christoph Hellwig Signed-off-by: Carlos Maiolino

x86/bugs: Add RSB mitigation document

2025-04-09T10:42:09Z

Create a document to summarize hard-earned knowledge about RSB-related mitigations, with references, and replace the overly verbose yet incomplete comments with a reference to the document. Signed-off-by: Josh Poimboeuf Signed-off-by: Ingo Molnar Link: https://lore.kernel.org/r/ab73f4659ba697a974759f07befd41ae605e33dd.1744148254.git.jpoimboe@kernel.org

x86/early_printk: Use 'mmio32' for consistency, fix comments

2025-04-09T10:27:08Z

First of all, using 'mmio' prevents proper implementation of 8-bit accessors. Second, it's simply inconsistent with uart8250 set of options. Rename it to 'mmio32'. While at it, remove rather misleading comment in the documentation. From now on mmio32 is self-explanatory and pciserial supports not only 32-bit MMIO accessors. Also, while at it, fix the comment for the "pciserial" case. The comment seems to be a copy'n'paste error when mentioning "serial" instead of "pciserial" (with double quotes). Fix this. With that, move it upper, so we don't calculate 'buf' twice. Fixes: 3181424aeac2 ("x86/early_printk: Add support for MMIO-based UARTs") Signed-off-by: Andy Shevchenko Signed-off-by: Ingo Molnar Reviewed-by: Denis Mukhin Link: https://lore.kernel.org/r/20250407172214.792745-1-andriy.shevchenko@linux.intel.com

Merge tag 's390-6.15-2' of git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux

2025-04-04T23:58:34Z

Pull more s390 updates from Vasily Gorbik: - Fix machine check handler _CIF_MCCK_GUEST bit setting by adding the missing base register for relocated lowcore address - Fix build failure on older linkers by conditionally adding the -no-pie linker option only when it is supported - Fix inaccurate kernel messages in vfio-ap by providing descriptive error notifications for AP queue sharing violations - Fix PCI isolation logic by ensuring non-VF devices correctly return false in zpci_bus_is_isolated_vf() - Fix PCI DMA range map setup by using dma_direct_set_offset() to add a proper sentinel element, preventing potential overruns and translation errors - Cleanup header dependency problems with asm-offsets.c - Add fault info for unexpected low-address protection faults in user mode - Add support for HOTPLUG_SMT, replacing the arch-specific "nosmt" handling with common code handling - Use bitop functions to implement CPU flag helper functions to ensure that bits cannot get lost if modified in different contexts on a CPU - Remove unused machine_flags for the lowcore * tag 's390-6.15-2' of git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux: s390/vfio-ap: Fix no AP queue sharing allowed message written to kernel log s390/pci: Fix dev.dma_range_map missing sentinel element s390/mm: Dump fault info in case of low address protection fault s390/smp: Add support for HOTPLUG_SMT s390: Fix linker error when -no-pie option is unavailable s390/processor: Use bitop functions for cpu flag helper functions s390/asm-offsets: Remove ASM_OFFSETS_C s390/asm-offsets: Include ftrace_regs.h instead of ftrace.h s390/kvm: Split kvm_host header file s390/pci: Fix zpci_bus_is_isolated_vf() for non-VFs s390/lowcore: Remove unused machine_flags s390/entry: Fix setting _CIF_MCCK_GUEST with lowcore relocation